The subject of the invention is a novel method for preparing a thin ceramic material with a continuous controlled surface porosity gradient and its use for producing electrochemical cells that conduct by oxide ions. This object is characterized by a continuous variation in porosity from 0% to about 80% of small thicknesses.
Porous ceramics have physico-chemical properties, whether thermal stability, chemical stability, biocompatability or mechanical strength, which make them good candidates for various applications such as filter membranes, sensors, ceramic-to-metal seals, biomaterials, energy conservation, thermal insulation or catalysis. These materials are used in particular for their low density, their high exchange area and their high permeability thanks to their open porosity.
As techniques for creating porosity in a ceramic, there are:                incomplete sintering of ceramic particles;        introduction of porosity by an emulsion of the material before sintering;        use of pore formers removed before sintering;        forming operations such as extrusion, injection molding, rapid prototyping; and        the use of ceramic fibers.        
These methods are listed in Roy W. Rice, “Porosity of ceramics”, Marcel Dekker, 1998, pp 20–21.
Incomplete sintering or subsintering of a ceramic powder or of a blend of ceramic powders having different particle sizes does not allow a porosity of greater than 50% to be achieved.
The use of pore formers, removed for example by pyrolysis before sintering, and leaving pores as the negative thereof in the ceramic, is one of the most appropriate methods for producing materials whose porosity is controlled in terms of volume fraction, shape and size distribution of the pores. Incorporating particulate pore formers, such as starch, lattices, graphite or resins into ceramic suspensions or slurries makes it possible to obtain uniformly distributed pores in a dense ceramic matrix. Depending on the forming method—pressing, casting in a mold, tape casting, extrusion or injection molding—a material is obtained with a plane geometry, a tubular geometry or a geometry of more complex shape.
Several embodiments of this technique of incorporating pore-forming particles into a ceramic suspension are disclosed in United States patents published under the numbers U.S. Pat. No. 4,777,153, U.S. Pat. No. 4,883,497, U.S. Pat. No. 5,762,737, U.S. Pat. No. 5,846,664 and U.S. Pat. No. 5,902,429 and in the publications by Lyckfeldt et al. and Apté et al. (O. Lyckfeldt, E. Lidén, R. Carlsson, “Processing of thermal insulation materials with controlled porosity”, Low Expansion Materials, pp 217–229; S. F. Corbin, P. S. Apté, J. Am. Ceram. Soc., 82, 7, 1999, pp 1693–1701). Apté et al. describe in particular a method using the tape casting of ceramic suspensions containing pore-forming particles and the thermocompression of the tapes in order to obtain, after sintering, a porous material with a discrete porosity gradient.
The pore former may also be a preform impregnated with a ceramic suspension (ceramic powder+solvent+organic additives).
The infiltration of polymer foams by a ceramic suspension is used to obtain bulk ceramics having a substantial open porosity. In this case, the total porosity is directly due to the structure of the foam, but this technique does not allow micron pore sizes to be achieved and cannot be used to prepare thin materials.
U.S. Pat. No. 4,780,437 discloses a method for preparing thin porous materials by infiltration of a flocking of pyrolyzable pore-forming fibers by a ceramic suspension. The materials obtained by this method have oriented anisotropic pores.
EP 0 212 230 discloses a method for preparing a ceramic filter, allowing a uniform porosity to be obtained throughout the filter.
Now, controlling the structure, whether as a dense system or a porous system with a porosity gradient, and controlling the microstructure, especially the particle size distribution and the pore size distribution of a ceramic article represents a key factor as regards its intrinsic properties and as regards its applications in terms of performance, reproducibility, lifetime and cost.
At the present time, it is not known how to manufacture a thin ceramic membrane, having a thickness of a few hundred microns, possessing a continuous controlled surface porosity gradient ranging from 0% (dense ceramic) to about 80% (highly porous system) in a single operation. All the articles produced using the various known methods have discrete or discontinuous controlled porosity gradients. Now, the presence, even in the same material, of these discrete porosity gradients may cause, at the various interfaces, layer debonding and delamination phenomena, especially because of the differences in thermal expansion coefficients between these regions. This results in rapid degradation of the article.
The fact of being able to produce a continuous controlled porosity gradient in a material should prevent the succession of interfaces between the layers of different porosity and consequently avoid these degradation phenomena.
In the production of electrochemical cells formed from a dense solid-state electrolyte and electrodes, called volume electrodes, such as those described in international patent application WO 95/32050, the fact of controlling a microstructure with a continuous controlled surface porosity gradient should make it possible:                to promote physical compatibility and chemical compatibility between volume electrode and dense solid-state electrolyte and thus improve the cohesion of the interface between these two materials;        to limit the energy costs associated with interfacial overpotentials; and        to promote the diffusion, disassociation and recombination of oxygen throughout the three-dimensional edifice of the volume electrode/dense solid-state electrolyte porous structure, by uniformly delocalizing volumewise the electrode reaction.        
The electrochemical cells thus formed have improved electrochemical performance in terms of electrochemical performance (current density applied per unit area), lifetime, aging and energy cost.